A fluidised bed typically consists of a bed of solid particles in the form of a powder (referred to as “media” or “solid media”) situated on a distributor plate located above a plenum chamber. The distributor plate has an arrangement of many gas flow passages through it. Introduction of air into the plenum chamber creates a pressure drop across the distributor plate. The resultant flow of process gas into the bed of media causes fluidisation. The result is a heterogeneous mixture of process gas and solid particles that behaves macroscopically as a fluid.
Fluidised beds typically provide a very high surface area contact between the fluidising gas and the solid media, compared with the contact area available for a packed solid bed. Fluidised beds also provide very good thermal transfer between the walls of the fluidised bed apparatus, the fluidising gas, the media and any component located in the media. This is due to the high surface area contact between the fluidising gas and the solid media and due to the very frequent particle-particle, particle-wall and particle-component collisions.
Fluidised bed apparatus typically have rectangular or cylindrical configurations.
It is known to use fluidised beds in order to provide a controlled heat treatment for components, for example in order to provide a hardness gradient within the component. Some example disclosures are discussed below.
U.S. Pat. No. 3,519,497 discloses a method of controlling the cooling of a rail section. The rail section is subjected to hot rolling and is immediately submerged in a fluidised bed. The fluidised bed is maintained at a predetermined temperature, in order to provide isothermal conditions for a bainitic microstructural transformation. The rail section is held in the fluidised bed in a particular orientation in order to provide stagnant regions of the flow in the fluidised bed. In turn, this affects the rate of cooling to which different parts of the rail section are subjected, and so affects the hardness/metallurgical properties throughout the rail section.
DE-C-3429707 discloses a method of locally hardening drill bits. A cartridge is loaded with drill bits. The cartridge is submerged into a fluidised bed. The cartridge holds the drill bits in such a way that, for each drill bit, only the surface to be treated is exposed whilst the remainder is shielded with insulation. This allows a custom boundary/interface to be achieved for varying component geometries.
In the two documents discussed above, the entire component is submerged in the fluidised bed, but special measures are taken in order to achieve differential heat treatment of different parts of the component.
Other documents disclose the submersion of only a part of the component to be treated, in order to ensure that only the submerged part is subjected to the required heat treatment. The intention here is also to provide a localised heat treatment in order to produce a controlled and sustained thermal gradient within and across the component.
For example, JP-A-2005-059054 discloses the use of a fluidised bed to create a high temperature gradient within a component. The component is partially dipped in at the top of the bed for localised heat treatment to induce a temperature gradient. FIGS. 2, 3 and 4 of JP-A-2005-059054 show how the component is suspended above the top of the bed and the part of the component to be treated is lowered into the bed.
JP-A-2003-013142 discloses heat treatment of a pipe section. The pipe section has a major portion formed with a constant, relatively small, wall thickness. A connection portion at the end of the pipe section, however, has a greater wall thickness. In order to apply the same heat treatment to the different parts of the pipe section, when taking into account the different wall thickness, the connection portion of the pipe is dipped into a fluidised bed, in order to provide a heat treatment specific to that part of the pipe section. The entire pipe section is held in a furnace in order to provide a heat treatment specific to the major portion of the pipe section.